1. Field of the Invention
The present invention relates to a combination of a cam and an adjusting shim used in a valve train in an internal combustion engine for automobiles.
2. Description of the Prior Art
In recent years, it has been strongly demanded that the fuel consumption of an automobile engine be improved by increasing the efficiency of the engine, and the reducing of a friction loss of an internal combustion engine has been studied as one of effective measures for solving this problem. It is said to be very effective to reduce the abrasion of contact surfaces of, especially, a cam and an adjusting shim in a valve train which are some of such sliding parts of an internal combustion engine, such as an automobile engine that are used under the severest conditions due to their low sliding speed and high load. The adjusting shim is a part for regulating a valve clearance, and has heretofore been formed out of a metal just as the cam.
It is generally said that a minimum clearance or a minimum thickness of an oil film between opposed sliding parts and the properties of sliding surfaces of the sliding parts have a great influence on the sliding characteristics thereof. As shown in, for example, "Hydraulic Pressure and Air Pressure" Vol. 18, No. 4, 1987, pages 247-258, and "Collection of Unprinted Theses Made Public in Scientific Lecture Meeting 924" edited by Society of Automobile Techniques, 1992, pages 85-88, an oil film parameter .LAMBDA. defined by the following equation 1 is used frequently as a value representing the measure of lubrication condition. EQU .LAMBDA.=h.sub.min .sigma.=h.sub.min /(R.sub.rms1.sup.2 +R.sub.rms2.sup.2).sup.1/2 (Equation 1)
wherein
h.sub.min is a minimum clearance or a minimum thickness of an oil film between opposed sliding parts, PA1 .sigma. is a composite surface roughness of opposed sliding parts, PA1 R.sub.rms1 is a roughness-root-mean square of a surface of one sliding part, and PA1 R.sub.rms2 is a roughness-root-mean square of a surface of the other sliding part.
It is said that values of this oil film parameter .LAMBDA. of not less than 3, not more than 1, and 1-3 indicate respectively a fluid lubrication condition, a boundary lubrication condition, and a mixed lubrication condition in which the fluid lubrication condition and boundary lubrication condition are seen in a mixed state, and that, as a value of .LAMBDA. becomes large, the contact between sliding surfaces is alleviated to cause the sliding characteristics of these surfaces to be improved. Therefore, since a minimum clearance or a minimum thickness h.sub.min of an oil film between the sliding parts under the same sliding conditions is constant, the minimizing of the roughness of the two sliding surfaces is effective in reducing the coefficient of friction thereof.
A method of minimizing the roughness of sliding surfaces of the sliding parts by subjecting these surfaces to a highly accurate super-precision finishing process is used in practice. However, it is difficult to apply a high-precision super precision finishing process to a complicatedly shaped surface, such as a curved surface like a surface of a cam, which is a part of a valve train, and, moreover, much time and labor are required, so that the machining cost becomes very high. Accordingly, a surface finishing process consisting of a regular grinding process is mainly used, and, therefore, the reducing of a coefficient of friction between a cam and a shim cannot be done satisfactorily at present.
In the meantime, a method of reducing a friction loss by smoothing rough surfaces of a cam and an adjusting shim has been proposed, in which the cam and adjusting shim are slidingly moved for this purpose without subjecting these parts to a high-precision super precision process. According to Japanese Patent Application Laid-Open No. 5-195723, increasing residual austenite on the sliding surface of an adjusting shim and forming a phosphate film on the surface of chill hardened cast iron of a cam cause the cam to polish and smooth the adjusting shim, and the cam surface which has been embrittled to be also broken and smoothed, so that the smoothing of the sliding surfaces progresses to enable a friction loss to decrease.
The inventors of the present invention also proposed the techniques for obtaining smooth sliding movements of an adjusting shim and a cam by employing a ceramic material for the production of the adjusting shim, and setting a ten-point mean roughness Rz of the sliding surface thereof to not more than 2.0 .mu.m (refer to Japanese Patent Application No. 3-179511, corresponding to U.S. Pat. No. 5,372,099), and the techniques for smoothing sliding surfaces during an initial period of an operation thereof by etching the sliding surface of an adjusting shim so as to embrittle the same, and thereby making the fine particles coming off from the embrittled surface polish a cam surface (refer to Japanese Patent Application No. 5-54962).
However, in the above-mentioned sliding surface smoothing techniques which utilize the sliding movements of a cam and an adjusting shim, the sliding surfaces are polished by the fine particles alone coming off due to the embrittlement and abrasion thereof. Therefore, there is a limit to the smoothing of these sliding surfaces, and, especially, it is impossible to maintain the surface roughness, the reduction of which is considered effective in reducing a friction loss, of the adjusting shim in a satisfactory stable specular condition (for example, a ten-point mean roughness Rz of not more than 0.1 .mu.m) for a long period of time.